Define stabilizing selection in biology

Define stabilizing selection in biology DEFAULT

B: Stabilizing, Directional, and Diversifying Selection

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Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.

Learning Objectives

  • Contrast stabilizing selection, directional selection, and diversifying selection.

Key Points

  • Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations.
  • In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.
  • Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
  • In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.

Key Terms

  • directional selection: a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction
  • disruptive selection: (or diversifying selection) a mode of natural selection in which extreme values for a trait are favored over intermediate values
  • stabilizing selection: a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population’s genetic variance will decrease.

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Stabilizing selection: Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

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Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

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Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

image

Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection

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A Simple Definition and Prominent Examples of Stabilizing Selection

Stabilizing Selection: Definition and Examples

Stabilizing selection is the process by which the intermediate form of a trait in an organism is selected and given preference over the extreme forms of that same trait, in order to maintain that common and adapted form in the population.

Stabilizing selection is the only type of natural selection that does not result in an adaptive change and/or evolution.

Natural selection is the gradual process by which the evolution of organisms occurs. If it is classified based on the observed effect on a particular trait, it can be divided into three distinct types, which are as follows.

  • Directional Selection
  • Disruptive Selection
  • Stabilizing Selection

Directional selection involves the selection of one extreme of any given trait over the other forms of the trait. It occurs when an organism experiences a change in its environmental conditions. Disruptive selection, on the other hand, involves favoring the extreme forms of a trait against the intermediate forms, thereby dividing the overall population into two distinct groups. Its occurrence depends on various factors involved in the species-habitat interaction. Stabilizing selection is the exact opposite of disruptive selection, as it favors the intermediate form of a trait against the extreme forms. It causes a reduction in the variation observed in the expression of that trait and allows the propagation of the intermediate, most adapted trait in the population. Over the course of time, this intermediate form becomes more and more common in the population, while the extreme forms become less common, till they are eventually lost.

Understanding Stabilizing Selection

❍ Stabilizing selection is defined as a type of natural selection that favors the propagation of the average or intermediate phenotype in a population, while actively selecting against the extreme phenotypes. It favors the major phenotype in a population that is well adapted to the environment. The quality of being average and well-adapted implies that it has been time-tested and has been proved as the most reliable and hardy phenotype, hence proving to be an advantage towards the survivability of the organism. This is the most common method of natural selection and takes place in all animals and plants.

❍ This type of selection, however, works to reduce the genetic diversity of the population, since the loss of extreme phenotypes also implies the loss of the genes and alleles coding for those phenotypes, it also conserves the form and functions of the phenotypic traits of the organisms over a period of time. This is evidenced in the unchanging anatomy of sharks and ferns. In most cases, this type of selection occurs for traits that are polygenic in nature, i.e. they are coded for by more than one gene. The interactions between these genes, give rise to various phenotypic outputs, of which the most adapted and common phenotype is actively selected. The other phenotypes are controlled by blocking the expression of a few select genes or by masking the expression of those genes.

❍ If a graph were to be drawn for such a selection, the distribution of traits in the original population would be represented by a normal bell shaped curve that showed a gradual increase followed by a gradual decrease in the incidence of a trait. If this curve were to undergo stabilizing selection, the selective forces would exert pressure over the extremities, and modifying the curve such that the narrow ends at both ends of the curve are eliminated, while the curved center is enhanced, giving rise to a tall and narrow curve. The narrowness implies the loss of the extreme phenotypes and the tallness of the curve indicated the increased incidence of the common/intermediate phenotype in the given population.

❍ This change in the shape of the curve can be explained by the example of coat hair color in rats. Consider a species of rats with a gradient of coat colors ranging from light brown, brown to dark brown, that inhabit a dense forest. In such a scenario, the forest floor would be covered with fallen and dried twigs and leaves and other sundry objects, causing the floor to gain a brownish appearance. Here, the intermediate rats with the brown coat color would escape detection by predators as their coat color would blend in with the color of the forest floor, thus acting as a sort of camouflage. However, in the case of the light and dark brown colored rats, their presence would be apparent and obvious, causing them to be easily preyed on. This would lead to an eventual decline in the prevalence of those phenotypes, leaving behind only the common and the most adapted phenotype of the brown coat in the population of the rats, thereby exhibiting stabilizing selection.

Examples of Stabilizing Selection

Baby on scales

❍ A classic example is that of the birth weight of human babies. If the birth weight is too low, the baby will be very weak and will experience considerable health problems. On the other hand, if it is too high, it will experience difficulty in passing through the birth canal, thereby inducing complications that could threaten the well-being of both the child as well as the mother. An average birth weight is the only scenario where the baby is born healthy without any problems, hence it is favored.

Robin with eggs

❍ The clutch size of birds is limited to a specific number of eggs to allow maximum survivability of the offspring. If the number of eggs are less, predation or affliction with a disease can easily wipe out the entire batch. Alternatively, if the clutch is too big, the large number of eggs will lead to the presence of more chicks to be fed, but the parent birds can only bring back a limited amount of food in their beaks, thus causing the chicks to starve and become malnourished. This is seen in the case of Robins.

Siberian husky runs

❍ The Siberian Husky is a breed of dogs that is well-suited to navigating dense snow-clad areas, due to its strong and well-defined pectoral and leg muscles. If these muscles were heavier, they would cause the dog to sink into the snow, and cause it to move slower or get stuck in the snow. On the other hand, if its muscles were any lighter, it would not be strong enough to pull sleds and other such equipment, rendering it quite useless as a working snow dog. Hence the muscle strength is an example of stabilizing selection.

Elephant eating cactus

❍ In the case of cacti, those with low spine-numbers are consumed by peccaries (wild, pig-like animals), causing the cacti to produce variants with high spine-number. But these too are not safe from consumption, as a parasitic insect prefers this variety of cacti for laying its eggs at the base of the spines. When these eggs hatch, the larvae feed on the fleshy stem of the plant and destroy it while completing their life-cycle. In order to prevent the decline of the cacti population, stabilizing selection eliminates the two extreme forms, leaving behind the intermediate form with the average spine-number, that just survives both types of predators.

Tea plantation

❍ This type is also seen in the case of plant height, where medium height is preferred over the extreme forms. If the plant is too short it may not receive enough sunlight to grow and proliferate, and if the plant is too tall it will perish due to wind damage. If the plant is of medium height, it is not only protected from wind damage but also receives plenty of sunlight for its growth.

Butterfly on leaf

❍ In case of flowers, moderate length of the nectary is selectively favored. If it is too long, the insect will not be able to reach into it, whereas if it is too short, the insect won&#;t come in contact with the anthers, and hence pollination would be hindered.

Despite the widespread occurrence of this type of selection, it is difficult to study due to the fact that its detection is highly complex. In order to determine whether a selection is the example of a stabilizing selection, one would have to study the mean and variance of traits in a population, the fitness of all the different naturally-occurring phenotypes, and the relation between the fitness values and the trait prevalence.

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The 3 Types of Natural Selection

Natural selection is defined as a process or a “force” that allows for organisms better adapted to their environment to better survive and produce more offspring. The theory of natural selection was first founded by Charles Darwin. The process of natural selection is important and is a driving force for evolution. For organisms to evolve, there needs to be differences in traits between organisms that provide certain advantages or disadvantages, and it is these traits that natural selection acts upon.

When it comes to natural selection, there are three different types of selection that can occur. These types include the following:

Stabilizing Selection

This type of natural selection occurs when there are selective pressures working against two extremes of a trait and therefore the intermediate or “middle” trait is selected for. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:

Example:  For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over. The plants that are very short fail to get enough sunlight to prosper. Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind.

Directional Selection

This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:



Example: Giraffes with the longest necks are able to reach more leaves to each. Selective pressures will work in the advantage of the longer neck giraffes and therefore the distribution of the trait within the population will shift towards the longer neck trait.

Disruptive Selection

This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait. This type of selection is not as common. When looking at a trait distribution, there are two higher peaks on both ends with a minimum in the middle as such:

Example: An area that has black, white and grey bunnies contains both black and white rocks. Both the traits for white and black will be favored by natural selection since they both prove useful for camouflage. The intermediate trait of grey does not prove as useful and therefore selective pressures act against the trait.

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12.3.1 Stabilizing Selection

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The enrichment of intermediate phenotypes is known as stabilizing selection. Examine the evolution of natural selection, explore examples of stabilizing selection and learn about the size and behaviors of stabilizing selection. Updated: 09/07/

Evolution by Natural Selection

Over many generations, the genetic makeup of organisms gradually changes due to evolution by natural selection. In a population of organisms, there is natural variation in genes and the phenotypes that they cause. For example, if we would look at the entire population of pigeons in a city, we'd see that there is a range of sizes, colors and willingness to be near humans. Many of these phenotypes are caused by genetic variations within the population.

Natural selection acts on phenotypic variation. Certain phenotypes can increase or decrease an organism's fitness, or ability to survive and reproduce. In our pigeon example, there may be reasons that a larger size or a browner color would improve pigeons' fitness. These reasons are called selective pressures, and what they are depends very much on the environment an organism finds itself in as well as the specific things it needs to do or avoid doing in order to survive until reproduction.

Individuals that are more fit are more likely to reproduce and pass their genes on to the next generation. Thus, genes that cause favorable phenotypes are selected for during natural selection, and genes that cause unfavorable phenotypes are selected against.

There are several major types of natural selection, such as directional selection, stabilizing selection, disruptive selection and sexual selection. The different types of selection lead to different overall changes in the population in the next generation. Here we'll focus on stabilizing selection.

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What Is Stabilizing Selection?

Stabilizing selection happens when extreme phenotypes on both ends of the spectrum are unfavorable. It's easier to understand stabilizing selection by looking at a visual representation. Here's a graph that describes this type of selection:

Stabilizing selection graph.

The top graph shows the natural variation in a given phenotype in the population before selection occurs. It's more or less a bell curve, with mostly intermediate phenotypes but also some extreme phenotypes at both ends of the spectrum. When stabilizing selection occurs, phenotypes at both extremes are selected against, and intermediate phenotypes are selected for. The bottom graph shows that the curve has become narrower and taller. It is narrower because there is less of a range of phenotypes, and it is taller because there are more organisms with intermediate phenotypes.

Examples of Stabilizing Selection

Stabilizing selection is likely the most common type of natural selection, but it's not always so easy to recognize it, because it doesn't lead to drastic changes in the way that directional selection or disruptive selection does. Let's look at some examples.

Number of Offspring

Let's take a concrete example like birds. The phenotype we are looking at is the number of eggs a mother bird lays. Let's say it ranges from a single egg to twenty eggs on the x-axis. Due to selective pressures in the birds' environment, like predators and scarcity of worms, both extremes of the spectrum get selected against. That means that, as evolution proceeds, there is a narrower range of phenotypes, and most pigeons lay somewhere between five and fifteen eggs, instead of between one and twenty.

Why? If a bird lays too few eggs, there's too great of a chance that predators will eat all of the eggs and her genes won't be passed on to the next generation. On the other hand, if she lays too many eggs, there's a high chance that she won't be able to find enough food to feed them all, and they could all starve, which again wouldn't allow her genes to be passed on.

The same thing happens with other animals that must care for their offspring, such as mammals. Too few offspring means that none may make it to reproduction, but on the other hand, the mother can't effectively care for too many offspring.

Size

There are many examples of stabilizing selection that have to do with organisms' sizes. A plant that has a very short stem may be selected against because it can't get enough light when other objects are nearby; however, a plant that has a very long stem may also be selected against because it can't support itself and is more easily damaged by wind and weather conditions.

In humans, birth weight is an example of stabilizing selection. Babies that are born too small can lose heat too easily and may die, whereas babies being born too large can lead to complications during childbirth and the death of the mother or the baby.

Behavior

Further examples of stabilizing selection come from organisms' behavioral phenotypes. Let's go back to our city pigeons from the beginning of the lesson. One of their variable phenotypes was their willingness to be near humans. We can imagine that, in this case, an intermediate phenotype would be selected for. If pigeons are too unafraid of humans, it would be dangerous for them because they could easily be injured by humans and their bicycles, cars and so forth; however, if they are too afraid of humans, they would miss out on valuable sources of food in the city.

Stabilizing selection can happen in cases of artificial selection, as well. Humans have been breeding dogs for thousands of years and selecting various phenotypes on purpose. If you wanted to breed guard dogs, you would select against both extremes of aggressiveness - dogs that are too aggressive would be too dangerous to live with, but guard dogs shouldn't be too tame, either.

Lesson Summary

In summary, stabilizing selection happens when extreme phenotypes on both ends of the spectrum are unfavorable. This type of selection leads to enrichment of intermediate phenotypes and narrowing of the range of variation.

Learning Outcomes

Upon completing this lesson, you should be able to:

  • Define fitness in terms of natural selection as well as selective pressures
  • Describe stabilizing selection and recall its importance
  • Identify examples of stabilizing selection

Stabilizing Selection: Fill-in-the-Blank Activity

This activity will help you assess your knowledge of the definition, examples, and graphical representation of stabilizing selection.

Guidelines

For this activity, print or copy this page on a blank piece of paper. Use the words presented in the word bank to complete the sentence. Neatly write them on the appropriate space provided.

Word Bank

Sentences

  1. __________ selection is the differential survival and reproduction of individuals due to a phenotypic variation.
  2. Selective pressure is external agents that change the behavior and __________ of an organism, depending on the given __________.
  3. Natural selection is a key process in the __________of a population in the next generation.
  4. An organism's __________ consists of observable traits, such as its appearance, behavior, and structure.
  5. __________ selection is the process where the __________ form of a trait in an organism is selected over the extreme phenotypes.
  6. The population before and after stabilizing selection is represented by a __________.
  7. In the field of evolution, fitness is not measured by how long an organism lives rather on how successful it is at __________.
  8. Stabilizing selection also applies to an organism's __________ phenotype.
  9. Stabilizing selection is a ubiquitous type of natural selection that doesn't lead to __________ changes in an organism.
  10. The eye color of an organism is an example of a phenotypic __________.

Answer Key

  1. Natural
  2. Fitness, Environment
  3. Evolution
  4. Phenotype
  5. Stabilizing, Intermediate
  6. Bell curve
  7. Reproducing
  8. Behavioral
  9. Drastic
  10. Trait

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  • Explain the different types of natural selection.
  • What are some examples of stabilizing selection?
  • Does stabilizing selection lead to genetic drift?
  • What is the formula for the bell curve?
  • How does the bell curve shift for stabilizing selection?
  • Lower birth weight babies are less likely to survive after birth, and high birth weight babies can cause complications during the birthing process. What type of selection (stabilizing, direction, or d
  • What are the advantages and disadvantages of using stabilization pond technology for purifying municipal wastewater?
  • Hyman P. Minsky&#;s work is related to his interpretation of Keynes, you should also talk to make the connection. Reference Stabilizing an Unstable Economy by Hyman P. Minsky
  • To overcome the problem of adverse selection, employers can use _____ techniques, such as _____. a. signaling, monitoring employee performance b. screening, monitoring employee performance c. scree
  • What type of natural selection (stabilizing, directional or disruptive) would the following statement be an example of and why: The typical number of eggs found in robins&#; nests if the environment of

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In define biology selection stabilizing

Stabilizing selection

Type of selection in evolution where a trait stabilizes around the average value

Three models for selection. In each panel, the curve with red color represents the population distribution before the occurrence of the corresponding selection and the curve with blue color represents the population distribution after the corresponding selection has occurred.

Stabilizing selection (not to be confused with negative or purifying selection[1][2]) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time.[3] Stabilizing selection commonly uses negative selection (a.k.a. purifying selection) to select against extreme values of the character. Stabilizing selection is the opposite of disruptive selection. Instead of favoring individuals with extreme phenotypes, it favors the intermediate variants. Stabilizing selection tends to remove the more severe phenotypes, resulting in the reproductive success of the norm or average phenotypes.[4] This means that most common phenotype in the population is selected for and continues to dominate in future generations.

Depending on the environmental conditions, a wolf may have an advantage over wolves with other variations of fur color. Wolves with fur colors that do not camouflage appropriately with the environmental conditions will be spotted more easily by the deer, resulting in them not being able to sneak up on the deer (leading to natural selection).

History[edit]

The Russian evolutionary biologist Ivan Schmalhausen founded the theory of stabilizing selection, publishing a paper in Russian titled "Stabilizing selection and its place among factors of evolution" in and a monograph "Factors of Evolution: The Theory of Stabilizing Selection" in [5][6]

Influence on population structure[edit]

Stabilizing selection causes the narrowing of the phenotypes seen in a population. This is because the extreme phenotypes are selected against, causing reduced survival in organisms with those traits. This results in a population consisting of fewer phenotypes, with most traits representing the mean value of the population. This narrowing of phenotypes causes a reduction in genetic diversity in a population.[7] Maintaining genetic variation is essential for the survival of a population because it is what allows them to evolve over time. In order for a population to adapt to changing environmental conditions they must have enough genetic diversity to select for new traits as they become favorable.[8]

Analyzing stabilizing selection[edit]

There are four primary types of data used to quantify stabilizing selection in a population. The first type of data is an estimation of fitness of different phenotypes within a single generation. Quantifying fitness in a single generation creates predictions for the expected fate of selection. The second type of data is changes in allelic frequencies or phenotypes across different generations. This allows quantification of change in prevalence of a certain phenotype, indicating the type of selection. The third type of data is differences in allelic frequencies across space. This compares selection occurring in different populations and environmental conditions. The fourth type of data is DNA sequences from the genes contributing to observes phenotypic differences. The combination of these four types of data allow population studies that can identify the type of selection occurring and quantify the extent of selection.[9]

However, a meta-analysis of studies that measured selection in the wild failed to find an overall trend for stabilizing selection.[10] The reason can be that methods for detecting stabilizing selection are complex. They can involve studying the changes that causes natural selection in the mean and variance of the trait, or measuring fitness for a range of different phenotypes under natural conditions and examining the relationship between these fitness measurements and the trait value, but analysis and interpretation of the results is not straightforward.[11]

Examples[edit]

The most common form of stabilizing selection is based on phenotypes of a population. In phenotype based stabilizing selection, the mean value of a phenotype is selected for, resulting a decrease in the phenotypic variation found in a population.[12]

Humans[edit]

Stabilizing selection is the most common form of nonlinear selection (non-directional) in humans.[13] There are few examples of genes with direct evidence of stabilizing selection in humans. However, most quantitative traits (height, birthweight, schizophrenia) are thought to be under stabilizing selection, due to their polygenicity and the distribution of the phenotypes throughout human populations.[14]

  • Birth Weight − A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.[15] The bell curve of the human population peaks at a birth weight that the newly born babies exhibit the minimum death rate.

Plants[edit]

  • Height − Another example of a trait, that might be acted on by stabilizing selection, is plant height. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.[16]
  • Cacti Spine Number − Desert populations of spiny cacti experience predation by peccaries, which consume the fleshy part of the cactus. This can be prevented by increasing the number of spines on the cactus. However, there is also a selection pressure in the opposite direction because there is a parasitic insect that will lay its eggs in spines if they are densely populated. This means that in order to manage both of these selection pressures the cacti experiences stabilizing selection to balance the appropriate number of spines to survive these different threats.[17]

Insects[edit]

  • Bicyclus anynana with wing eyespot, which experiences stabilizing selection to avoid predation.
    Butterfly's Winged Eyespots − The African butterfly Bicyclus anynana exhibits stabilizing selection with its wing eyespots.[18] It has been suggested that the circular eyespots positioned on the wings are favoured functionally compared to other shapes and sizes.[19]
  • Gall Size − The Eurosta solidaginis fly lays its eggs on the tip of plants, which then encase the larvae in a protective gall. The size of this gall is under stabilizing selection, as determined by predation. These larvae are under threat from parasitic wasps, which lay a single egg in galls containing the flies. The single wasp offspring then consumes the fly larvae to survive. Therefore, a larger gall is favored to allow more places for larvae to hide from the wasp. However, larger galls attract a different type of predation from birds, as they can penetrate large galls with their beak. Therefore, the optimal gall is moderately sized in order to avoid predation from both birds and wasps.[20]

Birds[edit]

  • Clutch Size − The number of eggs laid by a female bird (clutch size) is typically under stabilizing selection. This is because the female must lay as many eggs as possible to maximize the number of offspring. However, they can only lay as many eggs as they can support with their own resources. Laying too many eggs could expend all of the energy of the mother bird causing her to die and the death of the chicks. Additionally, once the eggs hatch the mother must be able to obtain enough resources to keep all of the chicks alive. Therefore, the mother typically lays a moderate amount of eggs in order to increase offspring survival and maximize the number of offspring.[21]

Mammals[edit]

  • The Siberian husky experiences stabilizing selection in terms of their leg muscles, allowing them to be strong but light.
    The Siberian husky experiences stabilizing selection in terms of their leg muscles. These dogs have to have enough muscle in order to pull sleds and move quickly. However, they also must be light enough to stay on top of the snow. This means that the leg muscles of the husky are most fit when they are moderately sized, to balance their strength and their weight.[22]

See also[edit]

References[edit]

  1. ^Lemey P, Salemi M, Vandamme A (). The Phylogenetic Handbook. Cambridge University Press. ISBN&#;.
  2. ^Loewe L. "Negative Selection". Nature Education. 1 (1):
  3. ^Charlesworth B, Lande R, Slatkin M (May ). "A neo-Darwinian commentary on macroevolution". Evolution; International Journal of Organic Evolution. 36 (3): – doi/jtbx. JSTOR&#; PMID&#; S2CID&#;
  4. ^Campbell NA, Reece JB (). Biology. Benjamin Cummings. pp.&#;–
  5. ^Levit GS, Hossfeld U, Olsson L (March ). "From the "Modern Synthesis" to cybernetics: Ivan Ivanovich Schmalhausen () and his research program for a synthesis of evolutionary and developmental biology". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. Wiley-Liss. (2): 89– doi/jez.b PMID&#;
  6. ^Adams MB (June ). "A Missing Link in the Evolutionary Synthesis. I. I. Schmalhausen. Factors of Evolution: The Theory of Stabilizing Selection". Isis. 79 (): – doi/ PMID&#; S2CID&#;
  7. ^Hunt J, Blows MW, Zajitschek F, Jennions MD, Brooks R (October ). "Reconciling strong stabilizing selection with the maintenance of genetic variation in a natural population of black field crickets (Teleogryllus commodus)". Genetics. (2): – doi/genetics PMC&#; PMID&#;
  8. ^"Low genetic variation". evolution.berkeley.edu. Retrieved
  9. ^Linnen CR, Hoekstra HE (). "Measuring natural selection on genotypes and phenotypes in the wild". Cold Spring Harbor Symposia on Quantitative Biology. 74: – doi/sqb PMC&#; PMID&#;
  10. ^Kingsolver JG, Hoekstra HE, Hoekstra J, Berrigan D, Vignieri SN, Hill CE, Hoang A, Gilbert P, Beerli P (). "The Strength of Super Genetic Selection in Natural Populations"(PDF). The American Naturalist. (3): – doi/ PMID&#; S2CID&#;
  11. ^Lande R, Arnold SJ (November ). "The Measurement of Selection on Correlated Characters". Evolution; International Journal of Organic Evolution. 37 (6): – doi/jtbx. PMID&#; S2CID&#;
  12. ^Kingsolver JG, Diamond SE (March ). "Phenotypic selection in natural populations: what limits directional selection?". The American Naturalist. (3): – doi/ PMID&#; S2CID&#;
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Sours: https://en.wikipedia.org/wiki/Stabilizing_selection
What is STABILIZING SELECTION? What does STABILIZING SELECTION mean? STABILIZING SELECTION meaning

Stabilizing Selection in Evolution

Stabilizing selection in evolution is a type of natural selection that favors the average individuals in a population. It is one of five types of selection processes used in evolution: The others are directional selection (which decreases the genetic variation), diversifying or disruptive selection (which shifts genetic variation to adjust to environmental changes), sexual selection (which defines and adapts to notions of "attractive" features of the individuals), and artificial selection (which is the deliberate selection by humans, such as that of the processes of animal and plant domestication).

Classic examples of traits that resulted from stabilizing selection include human birth weight, number of offspring, camouflage coat color, and cactus spine density.

Stabilizing Selection

  • Stabilizing selection is one of three main types of natural selection in evolution. The others are directional and diversifying selection. 
  • Stabilizing selection is the most common of those processes. 
  • The result of stabilizing is the over-representation in a specific trait. For example, the coats of a species of mice in a forest will all be the best color to act as camouflage in their environment. 
  • Other examples include human birth weight, the number of eggs a bird lays, and the density of cactus spines.

Stabilizing selection is the most common of these processes, and it's responsible for many of the characteristics of plants, humans and other animals.

Meaning and Causes of Stabilizing Selection

The stabilizing process is one that results statistically in an over-represented norm. In other words, this happens when the selection process—in which certain members of a species survive to reproduce while others do not—winnows out all the behavioral or physical choices down to a single set. In technical terms, stabilizing selection discards the extreme phenotypes and instead favors the majority of the population that is well adapted to their local environment. Stabilizing selection is often shown on a graph as a modified bell curve where the central portion is narrower and taller than the normal bell shape.

Diversity in a population is decreased due to stabilizing selection—genotypes which are not selected are reduced and can disappear. However, this does not mean that all individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are actually a bit higher statistically than those in other types of populations. This and other kinds of microevolution keep the "stabilized" population from becoming too homogeneous and allow the population the ability to adapt to future environmental changes.

Stabilizing selection works mostly on traits that are polygenic. This means that more than one gene controls the phenotype and so there is a wide range of possible outcomes. Over time, some of the genes that control the characteristic can be turned off or masked by other genes, depending on where the favorable adaptations are coded. Since stabilizing selection favors the middle of the road, a blend of the genes is often what is seen.

Examples of Stabilizing Selection

There are several classic examples in animals and humans of the results of stabilizing selection process:

  • Human birth weight, especially in underdeveloped countries and in the past of the developed world, is a polygenetic selection which is controlled by environmental factors. Infants with low birth weight will be weak and experience health problems, while large babies will have problems passing through the birth canal. Babies with average birth weight are more likely to survive than a baby that is too small or too large. The intensity of that selection has decreased as medicine has improved—in other words, the definition of "average" has changed. More babies survive even if they might have been too small in the past (a situation resolved by a few weeks in an incubator) or too large (resolved by Caesarian section).
  • Coat coloration in several animals is tied to their ability to hide from predator attacks. Small animals with coats that match their environments more closely are more likely to survive than those with darker or lighter coats: stabilizing selection results in an average coloration that's not too dark or too light.
  • Cactus spine density: Cacti have two sets of predators: peccaries which like to eat cactus fruits with fewer spines and parasitic insects which like cacti that have very dense spines to keep their own predators away. Successful, long-lived cacti have an average number of spines to help ward off both.
  • The number of offspring: Many animals produce multiple offspring at once (known as r-selected species). Stabilizing selection results in an average number of offspring, which is an average between too many (when there is a danger of malnourishment) and too few (when the chance of no survivors is highest).

Sources

Sours: https://www.thoughtco.com/types-of-natural-selection-stabilizing-selection

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Natural Selection and Adaptive Evolution

Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population.

Learning Objectives

Explain how natural selection leads to adaptive evolution

Key Takeaways

Key Points

  • Natural selection increases or decreases biological traits within a population, thereby selecting for individuals with greater evolutionary fitness.
  • An individual with a high evolutionary fitness will provide more beneficial contributions to the gene pool of the next generation.
  • Relative fitness, which compares an organism&#;s fitness to the others in the population, allows researchers to establish how a population may evolve by determining which individuals are contributing additional offspring to the next generation.
  • Stabilizing selection, directional selection, diversifying selection, frequency -dependent selection, and sexual selection all contribute to the way natural selection can affect variation within a population.

Key Terms

  • natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce
  • fecundity: number, rate, or capacity of offspring production
  • Darwinian fitness: the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype

An Introduction to Adaptive Evolution

Natural selection only acts on the population&#;s heritable traits: selecting for beneficial alleles and, thus, increasing their frequency in the population, while selecting against deleterious alleles and, thereby, decreasing their frequency. This process is known as adaptive evolution. Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce ( fecundity ), but if that same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism&#;s evolutionary fitness (or Darwinian fitness).

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Adaptive evolution in finches: Through natural selection, a population of finches evolved into three separate species by adapting to several difference selection pressures. Each of the three modern finches has a beak adapted to its life history and diet.

Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing additional offspring to the next generation and, thus, how the population might evolve.

There are several ways selection can affect population variation:

  • stabilizing selection
  • directional selection
  • diversifying selection
  • frequency-dependent selection
  • sexual selection

As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate. In the end, natural selection cannot produce perfect organisms from scratch, it can only generate populations that are better adapted to survive and successfully reproduce in their environments through the aforementioned selections.

Galápagos with David Attenborough: Two hundred years after Charles Darwin set foot on the shores of the Galápagos Islands, David Attenborough travels to this wild and mysterious archipelago. Amongst the flora and fauna of these enchanted volcanic islands, Darwin formulated his groundbreaking theories on evolution. Journey with Attenborough to explore how life on the islands has continued to evolve in biological isolation, and how the ever-changing volcanic landscape has given birth to species and sub-species that exist nowhere else in the world.

Stabilizing, Directional, and Diversifying Selection

Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.

Learning Objectives

Contrast stabilizing selection, directional selection, and diversifying selection.

Key Takeaways

Key Points

  • Stabilizing selection results in a decrease of a population &#;s genetic variance when natural selection favors an average phenotype and selects against extreme variations.
  • In directional selection, a population&#;s genetic variance shifts toward a new phenotype when exposed to environmental changes.
  • Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
  • In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.

Key Terms

  • directional selection: a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction
  • disruptive selection: (or diversifying selection) a mode of natural selection in which extreme values for a trait are favored over intermediate values
  • stabilizing selection: a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population&#;s genetic variance will decrease.

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Stabilizing selection: Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

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Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population&#;s genetic variance toward the new, fit phenotype.

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The Evolution of the Peppered Moth: Typica and carbonaria morphs resting on the same tree.The light-colored typica (below the bark&#;s scar) is nearly invisible on this pollution-free tree, camouflaging it from predators.

Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male&#;s territory. In this case, both the alpha males and the &#;sneaking&#; males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

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Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection

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Types of natural selection: Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

Frequency-Dependent Selection

In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.

Learning Objectives

Describe frequency-dependent selection

Key Takeaways

Key Points

  • Negative frequency -dependent selection selects for rare phenotypes in a population and increases a population&#;s genetic variance.
  • Positive frequency-dependent selection selects for common phenotypes in a population and decreases genetic variance.
  • In the example of male side-blotched lizards, populations of each color pattern increase or decrease at various stages depending on their frequency; this ensures that both common and rare phenotypes continue to be cyclically present.
  • Infectious agents such as microbes can exhibit negative frequency-dependent selection; as a host population becomes immune to a common strain of the microbe, less common strains of the microbe are automatically favored.
  • Variation in color pattern mimicry by the scarlet kingsnake is dependent on the prevalence of the eastern coral snake, the model for this mimicry, in a particular geographical region. The more prevalent the coral snake is in a region, the more common and variable the scarlet kingsnake&#;s color pattern will be, making this an example of positive frequency-dependent selection.

Key Terms

  • frequency-dependent selection: the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population
  • polygynous: having more than one female as mate

Frequency-dependent Selection

Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).

Negative Frequency-dependent Selection

An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males are the smallest and look a bit like female, allowing them to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. The big, strong orange males can fight off the blue males to mate with the blue&#;s pair-bonded females; the blue males are successful at guarding their mates against yellow sneaker males; and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males.

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Frequency-dependent selection in side-blotched lizards: A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.

In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes. In one generation, orange might be predominant and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for.Finally, when blue males become common, orange males will once again be favored.

An example of negative frequency-dependent selection can also be seen in the interaction between the human immune system and various infectious microbes such as pathogenic bacteria or viruses. As a particular human population is infected by a common strain of microbe, the majority of individuals in the population become immune to it. This then selects for rarer strains of the microbe which can still infect the population because of genome mutations; these strains have greater evolutionary fitness because they are less common.

Positive Frequency-dependent Selection

An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. The scarlet kingsnake, a harmless species, mimics the coloration of the eastern coral snake, a venomous species typically found in the same geographical region. Predators learn to avoid both species of snake due to the similar coloration, and as a result the scarlet kingsnake becomes more common, and its coloration phenotype becomes more variable due to relaxed selection. This phenotype is therefore more &#;fit&#; as the population of species that possess it (both dangerous and harmless) becomes more numerous. In geographic areas where the coral snake is less common, the pattern becomes less advantageous to the kingsnake, and much less variable in its expression, presumably because predators in these regions are not &#;educated&#; to avoid the pattern.

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Lampropeltis elapsoides, the scarlet kingsnake: The scarlet kingsnake mimics the coloration of the poisonous eastern coral snake. Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.

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Micrurus fulvius, the eastern coral snake: The eastern coral snake is poisonous.

Negative frequency-dependent selection serves to increase the population&#;s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.

Sexual Selection

Sexual selection, the selection pressure on males and females to obtain matings, can result in traits designed to maximize sexual success.

Learning Objectives

Discuss the effects of sexual dimorphism on the reproductive potential of an organism

Key Takeaways

Key Points

  • Sexual selection often results in the development of secondary sexual characteristics, which help to maximize a species &#; reproductive success, but do not provide any survival benefits.
  • The handicap principle states that only the best males survive the risks from traits that may actually be detrimental to a species; therefore, they are more fit as mating partners.
  • In the good genes hypothesis, females will choose males that show off impressive traits to ensure they pass on genetic superiority to their offspring.
  • Sexual dimorphisms, obvious morphological differences between the sexes of a species, arise when there is more variance in the reproductive success of either males or females.

Key Terms

  • sexual dimorphism: a physical difference between male and female individuals of the same species
  • sexual selection: a type of natural selection, where members of the sexes acquire distinct forms because members choose mates with particular features or because competition for mates with certain traits succeed
  • handicap principle: a theory that suggests that animals of greater biological fitness signal this status through a behavior or morphology that effectively lowers their chances of survival

Sexual Selection

The selection pressures on males and females to obtain matings is known as sexual selection. Sexual selection takes two major forms: intersexual selection (also known as &#;mate choice&#; or &#;female choice&#;) in which males compete with each other to be chosen by females; and intrasexual selection (also known as &#;male–male competition&#;) in which members of the less limited sex (typically males) compete aggressively among themselves for access to the limiting sex. The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision.

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Sexual selection in elk: This male elk has large antlers to compete with rival males for available females (intrasexual competition).Tn addition, the many points on his antlers represent health and longevity, and therefore he may be more desirable to females (intersexual selection).

Sexual Dimorphism

Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. Males are often larger, for example, and display many elaborate colors and adornments, such as the peacock&#;s tail, while females tend to be smaller and duller in decoration. These differences are called sexual dimorphisms and arise from the variation in male reproductive success.

Females almost always mate, while mating is not guaranteed for males. The bigger, stronger, or more decorated males usually obtain the vast majority of the total matings, while other males receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to obtain those matings, resulting in the evolution of bigger body size and elaborate ornaments in order to increase their chances of mating. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.

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Sexual dimorphism: Morphological differences between males and females of the same species is known as sexual dimorphism.These differences can be observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and (c) wood ducks.

Sexual dimorphism varies widely among species; some species are even sex-role reversed. In such cases, females tend to have a greater variation in their reproductive success than males and are, correspondingly, selected for the bigger body size and elaborate traits usually characteristic of males.

The Handicap Principle

Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual&#;s survival, even though they maximize its reproductive success. For example, while the male peacock&#;s tail is beautiful and the male with the largest, most colorful tail will more probably win the female, it is not a practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place. Because large tails carry risk, only the best males survive that risk and therefore the bigger the tail, the more fit the male. This idea is known as the handicap principle.

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A male bird of paradise: This male bird of paradise carries an extremely long tail as the result of sexual selection.The tail is flamboyant and detrimental to the bird&#;s own survival, but it increases his reproductive success.This may be an example of the handicap principle.

The Good Genes Hypothesis

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be so selective because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.

BBC Planet Earth &#; Birds of Paradise mating dance: Extraordinary Courtship displays from these weird and wonderful creatures. From episode 1 &#;Pole to Pole&#;. This is an example of the extreme behaviors that arise from intense sexual selection pressure.

No Perfect Organism

Natural selection cannot create novel, perfect species because it only selects on existing variations in a population.

Learning Objectives

Explain the limitations encountered in natural selection

Key Takeaways

Key Points

  • Natural selection is limited by a population &#;s existing genetic variation.
  • Natural selection is limited through linkage disequilibrium, where alleles that are physically proximate on the chromosome are passed on together at greater frequencies.
  • In a polymorphic population, two phenotypes may be maintained in the population despite the higher fitness of one morph if the intermediate phenotype is detrimental.
  • Evolution is not purposefully adaptive; it is the result of various selection forces working together to influence genetic and phenotypical variances within a population.

Key Terms

  • linkage disequilibrium: a non-random association of two or more alleles at two or more loci; normally caused by an interaction between genes
  • genetic hitchhiking: changes in the frequency of an allele because of linkage with a positively or negatively selected allele at another locus
  • polymorphism: the regular existence of two or more different genotypes within a given species or population

No Perfect Organism

Natural selection is a driving force in evolution and can generate populations that are adapted to survive and successfully reproduce in their environments. However, natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it cannot create anything from scratch. Therefore, the process of evolution is limited by a population&#;s existing genetic variance, the physical proximity of alleles, non-beneficial intermediate morphs in a polymorphic population, and non-adaptive evolutionary forces.

Natural Selection Acts on Individuals, not Alleles

Natural selection is also limited because it acts on the phenotypes of individuals, not alleles. Some alleles may be more likely to be passed on with alleles that confer a beneficial phenotype because of their physical proximity on the chromosomes. Alleles that are carried together are in linkage disequilibrium. When a neutral allele is linked to beneficial allele, consequently meaning that it has a selective advantage, the allele frequency can increase in the population through genetic hitchhiking (also called genetic draft).

Any given individual may carry some beneficial alleles and some unfavorable alleles. Natural selection acts on the net effect of these alleles and corresponding fitness of the phenotype. As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; similarly, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.

Polymorphism

Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency because the intermediate morph is detrimental.

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Polymorphism in the grove snail: Color and pattern morphs of the grove snail, Cepaea nemoralis.The polymorphism, when two or more different genotypes exist within a given species, in grove snails seems to have several causes, including predation by thrushes.

For example, consider a hypothetical population of mice that live in the desert. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of black rock. The dark-colored mice may be more fit than the light-colored mice, and according to the principles of natural selection the frequency of light-colored mice is expected to decrease over time. However, the intermediate phenotype of a medium-colored coat is very bad for the mice: these cannot blend in with either the sand or the rock and will more vulnerable to predators. As a result, the frequency of a dark-colored mice would not increase because the intermediate morphs are less fit than either light-colored or dark-colored mice. This a common example of disruptive selection.

Not all Evolution is Adaptive

Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and gene flow, often do the opposite by introducing deleterious alleles to the population&#;s gene pool. Evolution has no purpose. It is not changing a population into a preconceived ideal. It is simply the sum of various forces and their influence on the genetic and phenotypic variance of a population.

Sours: https://courses.lumenlearning.com/boundless-biology/chapter/adaptive-evolution/


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